In the early 1960's aerial application of fire retardant compositions, to prevent or retard the spread of forest fires, range fires, etc., became very widespread. Typically, these fire retardant compositions contained an electrolytic fire suppressing salt such as ammonium phosphate, ammonium sulfate, and the like and also included other components such as viscosity modifiers, corrosion inhibitors and coloring agents such as pigments or dyes. Typical fire retardant compositions of the type described above are disclosed in the patents to Nelson, U.S. Pat. No. 3,196,108, and to Langguth et al., U.S. Pat. Nos. 3,257,316 and 3,309,324. These compositions generally consisted of an aqueous slurry or solution of a fire suppressing salt such as ammonium phosphate or ammonium sulfate and a thickening agent such as attapulgite clay, guar gum or the like. Coloring agents such as red iron oxide were included to improve the visibility of the material after it was dropped. More recently, ammonium polyphosphate liquids, containing coloring agents and corrosion inhibitors, have been widely employed. Such liquid polyphosphate fire retardant compositions are disclosed in the patents to Nelson, U.S. Pat. No. 3,370,890 and to Lacey, U.S. Pat. No. 3,960,735.
Fire retardant compositions containing other thickeners, stabilizers and the like are disclosed in the patents to Strickland, U.S. Pat. No. 4,822,524; Morganthaler, U.S. Pat. No. 3,634,234; Kegler et al., U.S. Pat. No. 4,606,831; Vandersall, U.S. Pat. No. 4,447,336; Adl et al., U.S. Pat. No. 4,447,338; and Vandersall, U.S. Pat. Nos. 4,839,065 and 4,983,326.
Fire retardant compositions are typically manufactured as dry or liquid "concentrates". These concentrates are shipped and stored in such form until just prior to use. Then, the concentrate is mixed with water to form the final diluted "mixed" fire retardant composition. This mixed retardant composition is then pumped in the tanker aircraft for transport to and dropping at the wildfire site. In some instances a dry concentrate is first mixed with an initial quantity of water to provide an intermediate liquid concentrate and this intermediate liquid concentrate is then further diluted just before use to the final diluted mixed form.
The active fire suppressing components employed in such retardant compositions include any of the well known electrolytic fire suppressing salts, e.g., such as are disclosed in the patent to Nelson U.S. Pat. No. 3,196,108, as well as the more recently employed liquid ammonium polyphosphate materials, as disclosed in the patent to Lacey U.S. Pat. No. 3,960,735; ammonium sulfate, as disclosed in the patent to Crouch U.S. Pat. No. 4,176,071; and mixtures of these salts with themselves and with other salts.
In general, the active fire retardant components are compounds or a mixture of compounds that degrade or decompose at temperatures below the ignition temperature of the fuels to be protected (e.g., cellulose), thereby releasing a mineral acid, such as phosphoric acid or sulfuric acid. Among the various fire retardants typically used in fire retardant mixtures and which might be used in the compositions of this invention are monoammonium orthophosphate, diammonium orthophosphate, monoammonium pyrophosphate, diammonium pyrophosphate, triammonium pyrophosphate, tetraammonium pyrophosphate, ammonium polyphosphate, substituted ammonium polyphosphate, amide polyphosphate, melamine polyphosphate, ammonium-alkali metal mixed salts of orthophosphate, ammonium-alkali metal mixed salts of pyrophosphate, ammonium-alkali metal mixed salts of polyphosphate, ammonium-alkaline earth metal mixed salts of orthophosphate, ammonium-alkaline earth metal mixed salts of pyrophosphate, ammonium-alkaline earth metal mixed salts of polyphosphate, ammonium sulfate, liquid ammonium polyphosphates and blends thereof. Some liquid ammonium polyphosphates may be too dilute in their commercial forms for application as fire retardants but, other retardants, such as those noted above, may be mixed with a liquid ammonium polyphosphate until a minimum acceptable concentration is obtained. Ammonium polyphosphate is often called polyammonium phosphate, and commonly contains other ammonium phosphate such as pyro and metaphosphates, and the alkali metal equivalents thereof, as well as a blend of phosphate polymers. Such polyammonium phosphates are often referred to as 10-34-0, 11-37-0, 12-40-0, 13-42-0 or the like, where the first number indicates the percentage of nitrogen in the blend, the middle number indicates the percentage phosphate in the blend and the last number indicates the percentage potash in the blend.
The fire retardant components may also include thickening agents, which include standard thickeners such as galactomannan guar gum compositions and derivatives thereof attapulgite clay, carboxymethylcellulose and derivatives thereof, and the like. The thickening agent is employed to maintain the viscosity of the diluted mixed fire retardant composition, for example, at between about 50 centipoise and about 2000 centipoise for aerial application. In addition, the fire retardant components, in the concentrate or in the final diluted mixed form, may also typically include various adjuvants such as corrosion inhibitors, flow conditioners, spoilage inhibitors, stabilizers and the like, and carriers for these adjuvants, in accordance with art recognized principles.